Dell Gaming Lite Backpack 17 is part of eBay Dell sale, with coupon code comes down to $29.75.
It can fit 17'' laptop, with spare room for headset, mouse, charger, cables, etc.
Or you can fit books, notepads, snacks and random junk instead of laptop.
Water-resistant fabric, padding on all sides, additional side pockets for e.g. water bottle and umbrella.
RRP is $62. More details on product page on Dell's site:
Edit: This was sold out overnight (over 200 sold), but is back in stock now.